Ball nut screw assembly



Jan. 29, 1957 W, H DOERFNER 2,779,206

BALL NUT SCREW ASSEMBLY Filed Aug. 18, 1955 Mfr/fag@ ATTORNEY UnitedStates Patent BALL NUT SCREW ASSEMBLY William H. Doerfner, Saginaw,Mich., assigner to General Motors Corporation, Detroit, Mich., acorporation of Delaware Application August 18, 1955, Serial No. 529,150

5 Claims. (Cl. 74-459) My invention relates to worm and nut `devices ofthe type wherein the connection between the worm and nut is a train ofballs confined within a passageway formed by the helical grooves in theworm and nut.

`Devices of this type find application, for example, in machine toolsand in various types of actuators, including aircraft actuators. Alsosuch a device may be used, with advantage, in steering mechanisms, bothmechanical and powered-sea for instance, Hawkins Patent 2,267,524 andco-pending application Serial No. 431,422 filed May 2K1, 1954, in thenames of Clovis W. Lincoln, Henry S. Smith, and Philip B. Zeigler.

While the present invention is generally applicable to ball bearingscrew `and nut devices, it represents an outgrowth of diflicultiesexperienced during the development of the power steering gear disclosedin the identiiied application. This gear is distinguished inter alia inthat the power piston is housed within the gear box and in that thepiston carries rack teeth mating with the gear sector integral with thecrossl shaft and connecting through the shaft with the pitrnan arm. Thepiston is bored and counterbored to accommodate the worm end of thesteering shaft and the ball nut, the latter being positioned in thecounterbore in xed relation to the piston and surrounding the worm.

It was found on an initial test of the described gear that thereversibility thereof was unsatisfactory due largely to the faulty lashcharacteristics of the worm and nut assembly. These faulty lashcharacteristics derived from the fact that it is virtually impossiblewith present day grinding equipment to provide a helical groove in apiece of stock, which does not taper to some extent from one end of thepiece to the other. This problem had not been of particular importanceas to prior gears, where the lash between the worm and nut could readilybe set for any relative position of these parts, despite an imperfectworm groove, through controlsapplied in the generation of the gearsector teeth and the mating rack teeth (carried by the nut in theseprior constructions) and through axial adjustment of the gear sectorrelative to the nut after assembly. In the power steering gear ofLincoln et al. the nut, as brought out above, does not mate ydirectlywith the gear sector and it, therefore, was not possible to handle thelash problem in the manner just indicated.

It should be here injected that for good reversibility in a ball nuttype steering gear, it is highly desirable, if not essential, that therebe no lash between the worm and nut when the same are in centeredrelation and that the play or lash between the parts off-centerprogressively increase in either direction. With the lash so adjusted,there is little, if any, likelihood of overshooting beyond center on thereversal, this because of the braking action afforded by the no-lashcondition over center.

ln accordance with the present invention I achieve the desideratum justdescribed by gradually varying the pitch diameter of the thread of oneor both of the worm and 2,779,206 Patented Jan. 29,1957

Z the nut outward of center. Most preferably, and largely as a matter ofconvenience, I gradually vary the pitch diameter of the worm threadrather than the internal thread of the nut.

On reection, it will be recognized that in effect, in-

' stead of seeking to avoid the formation of a tapered groove in thegrinding operation yielding the Worm, as has been'the practiceheretofore, I deliberately introduce taper, and yet l solve the problemwhich has been occasioned by tapering.

My invention will be better understood by reference to the accompanyingdrawings and the detailed description which follows. =In the drawings,

Figure l shows a typical ball nut and screw assembly;

Figure 2 is a detail of the worm or screw component, the eifect of myinvention on the shape thereof being exaggerated; and

Figure 3 is an enlarged View showing one of the connecting 4balls seatedin the helical worm groove.

In Figure l, the worm and nut are denoted by the numerals 1t) and 12,respectively. The nut carries a key 14 whereby it may be connected to apart (notshown) to be actuated. Otherwise, the nut is adequatelydescribed in the previously mentioned Hawkins Patent I2,267,524, and inview of such patent a detailed description here is consideredunnecessary. Suii'ice it to say that the nut has a helical internalgroove complementary to the worm groove and that it encases two separateball trains as reflected by the two return tubes 16 and 17.

-A worm or screw ground or otherwise formed according to the instantinvention has a slightly barrel shaped pitch line configuration, anelect which, as before indicated, is exaggerated in Figure 2 in theinterest of simplifying an understanding of the invention. Actually, aworm of the size employed would not appear so shaped to the naked eye.

The barrel shape results from the progressive decrease in the pitchdiameter toward each end as compared with that at the center C-L of theworm. Pitch diameter may be defined as twice the distance between theaxis of the worm and the mean point of contact of the ball 18 i(Fig. 43)and the side wall of thread 20, i. e. twice the distance a--b.

'In forming the particular worm here in contemplation, I avoid anyvariance in the pitch diameter over 300-360 of lead to each side of thecenter dine. This gives the desired no-lash condition of the worm andnut when the same are in their centered position. Also, in the case ofthe particular worm, I iind it advantageous to obtain the gradual pitchdiameter reduction by progressively increasingthe depth of the over-allprofile of the worm groove 22 relative to the external diameter of theworm at a rate of the order of .003/.004 inch per linear inch of worm.

Having thus described and illustrated my invention what I claim is:

l. 'In a worm and nut device wherein the connection between the worm andnut comprises a ball train coniined in a passageway formed by the wormgroove andi a complementary internal helical groove in the nut,theimprovement which resides in gradually varying the pitch. diameter ofthe thread of one or both of said worm and.; said nut 4outward of centerso as to obtain a condition of no-lash on center and progressivelyincreasing lash off-center.

2. In a worm and nut device wherein the connection between the worm andnut comprises a ball train conned in a passageway formed bythe wormgroove and a complementary internal helical groove in the nut, the

improvement which resides in gradually varying the pitch` diameter ofthe thread of one of said worm and said nut in either direction awayfrom center so as to obtain:

a condition of 11o-lash on 'center and progressively increasing .lasholf-center.

3. In a worm and nut device wherein the connection between the worm andnut comprises a ball train conned in a .passageway formed by vthe wormgroove and a complementary internal vhelical groove in the nut, theimprovement which resides in gradually varying the pitch diameter of thethread of the worm outward of center in both directions so that inrapplication of the device there :is no lash between the worm and nutwhen the same are in centered relation and so that no relative movementof the worm and nut ol center in either direction a condition ofprogressively increasing lash obtaine.

4. A device according to claim 3 in which the depth of the worm grooveprogressively increases in either direction off-center at a rate of theorder of .GOS/.O04 inch per linear inch.

5. A device according to claim 3 wherein the no-lash condition obtainsto the extent of from about 300 to about 360 of llead both sides-ofcenter.

No references cited,

